Field emission scanning microscope display

ABSTRACT

A field emission scanning microscope display system wherein a field emission gun generates a beam of charged particles which impinge upon a specimen to be investigated. A detector of the scintillation type produces an image signal relative to the impingement of the beam on the specimen. The image signal is appropriately amplified and conducted to a signal grid of a television type viewing monitor. A control unit synchronously deflects the microscope beam and the electron beam of the viewing tube in a predetermined pattern. The pattern is selected to have preferably a four-to-one interlacing and the interlacing sequence is selected so as to render the image formed on the viewing tube substantially stationary in a normal viewing mode.

United States Patent Coates et al.

[ Get. 23, R973 FIELD EMISSION SCANNING MICROSCOPE DISPLAY I lInventors: Vincent J. Coates, Los Altos;

Leonard M. Welter, Saratoga, both of Calif.

[73] Assignee: American Optical Corporation,

Southbridge, Mass.

[22] Filed: May 8, 1972 [21] Appl. No.: 251,125

[52] US. Cl. 250/310, 250/311, l78/DIG. 3 [51] Int. Cl. H01j 37/26 [58]Field of Search 250/495 A, 49.5 PF,

I 250/310, 311; l78/DIG. 3

[56] References Cited UNITED STATES PATENTS 3,309,461 3/l967 Deutsch....l78/DlG.3 2,940,005 6/1960 Toulon l78/DIG. 3

OTHER PUBLICATIONS Pulse and Digital Circuits" Millman et al., McGrawHill, 1956 pp. 515-517. I

Primary Examiner-James W. Lawrence Assistant Examiner-B. C. AndersonAttorney-William C. Nealon et al.

[57] ABSTRACT A field emission scanning microscope display systemwherein a field emission gun generates a beam of charged particles whichimpinge upon a specimen to be investigated. A detector of thescintillation type produces an image signal relative to the impingementof the beam on the specimen. The image signal is ap-- 7 Claims, 3Drawing Figures l6 ACCELERATION VOLTAGE Q? TIP VOLTAGE so l2 7 1 I l v51s SCAN 33 ifi$ AND, 34

SYNCH d CONTROL ZOT 31 22 X [i DETECTOR PATENTEDUBI 23 mm ACCELERATION'VOLTAGE TIP VOLTAGE SCAN AND SYNCH CONTROL DETECTOR FIG. i

FTGB

FIG. 2

FIELD EMISSION SCANNING MICROSCOPE DISPLAY BACKGROUND OF THE INVENTIONThe features of the applicants invention are subject to a wide range ofapplication, however, they are especially suited for use in a fieldemission scanning microscope display system and will therefore beparticularly described in that connection.

One of the very significant advantages of scanning electronmicroscopesystems involves the ability to directly view the specimen ona cathode ray tube-type monitor. This real time viewing of the specimenpermits the practitioner to gain much valuable information even thoughthe full resolution of the device may only be achievable on aphotographic record of the image. In scanning electron microscopes ofother than the field emission cathode type,the intensity of the beam isseverely limited and thus the signal generated from the surface of thespecimen by secondary'electrons, reflected electrons, or other emittedparticles is of a con comitantly low value. To enable real time viewingon a cathode ray tube, it is essential that a slow mode scanning of thespecimen and the tube'be utilized to assure the necessarysignal-to-noiseratio for intelligible transmission of information.

' With the advent of the field emission type scanning electronmicroscope, the achievable high beam intensity permits rapid scanning ofthe specimen and the monitor in a manner similar to normal televisionpractices. The user may comfortably view the image display on a videomonitor without the .use of extremely high persistence screens and theresultant loss in viewing information and resolution. In a slow scanmicroscopy system, the same ease of viewing and information would onlybe attainable through the use of an ancillary video-type tapereproduction system.

The raster of the monitor appears and is entirely similar to that of thenormal television receiver system. In

,the United States, the accepted'and approved television standardsspecify a 525 line scanning system having a'two-to-one interlace ratio.While 525 lines areentirely adequate at a normal viewing distance for TVrec'eption, to achieve line-free photographs and better resolution as inscanning electron miscroscopy systems, it is'necessary to go to a highernumber of scanning lines. The higher number of lines enormously expandson the specimen. A viewing tube operatively associated with the detectordisplays an image of the specimen upon a sensitized face of the tube.The image is produced by modulation of the viewing tube electron beam inaccordance with the image signal. A control unit synchronously deflectsthe charged particle beam as well as the viewing tube beam in apredetermined scanning pattern. The pattern is selected to have at leasta three-to-one interlacing. The sequence of interlacing with respect tothe separate fields making up individual frames of the picture isselected to render the viewing tube image sustantially stationary in anormal operator viewing mode.

Since the bandwidth is proportional to the square of the number ofpicture elements, approximately doubling the number of lines to achievethe necessary resultion,'necessitates a four-to-one increase inbandwidth. To limit this bandwidth requirement,realizing that it is alsodirectly proportional to the number of interlace ratio greater thantwo-to-one, as, for example,

the required bandwidth of the display system and sig- SUMMARY OF THEINVENTION Briefly, the applicants invention contemplates a fieldemission scanning microscope display system. A field emission gungenerates a .beam of charged particles which are accelerated and focusedto impinge upon a specimen undergoing investigation. A detector suitablylocated with respect to the specimen produces an image signalrelative tothe impingement of the beam a four-to-one ratio, it is found that anormal sequential field scanning pattern produces a cascading effect tothe eye of the observer. The visual, physiological, and psychologicalfactors producing this waterfall effect to the eye of the observer isnot understood. However, without being bound by the proposed theory, itis hy pothesized that as each line of succeeding fields are painted onthe face of the tube in next physical order, the eye tends to followsuch lines thereby presenting an apparently moving image in either avertically up or down direction depending upon the direction of motionor placement of the scanning line. The applicants novel contribution tothe art overcomes this problem as well as limiting the bandwidthrequirements of the system.

For a better understanding of the present invention together with otherand further objects thereof,'reference is had to the followingdescription of the preferred embodiment taken in connection withaccompanying drawings, its scope is pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic-schematicview of a field emission scanning electron microscope display system;

FIG. 2 is a partial diagrammatic representation of a viewing rasterdemonstrating the scanning pattern of the applicants invention;

FIG. 3 is a partial diagrammatic representation of a viewing rasterutilizing a normal scanning pattern having a four-to-one interlaceratio.

The drawings as well as the accompanying description are intended to beillustrative of the applicants invention and in no way delimiting of itsscope.

DESCRIPTION OF THE PREFERRED EMBODIMENT The scanning miscroscopy displaysystem of FIG. 11 operates a field emission electron gun It)interconnected to a specimen chamber 20. The necessary operatingvoltages for electron gun are supplied by tip voltage unit 17 andacceleration voltage unit 16. Scan and Sync control unit 32, detectorunit 31, and the viewing tube monitor 30 comprise the remaining portionof the display system. 7

Field emission gun 10 is typical of those presently found in the art andreference may be had to an article appearing in The Review Of ScientificInstruments, Volume 39, Number 4, April 1968 entitled ELEC- TRON GUNUSING A FIELD EMISSION SOURCE, authors A. V. Crewe, D. N. Eggenberger,.1. Wall, and L. M. Welter, for a more complete explanation of itsoperating characteristics and parameters. In further reference to thistype of microscopy instrument, the applicants co-pending applicationSer. No. 46,425 filed June 15,1970, and now U. S. Pat. No. 3,678,333entitled FIELD EMISSION ELECTRON GUN, which is hereby incorporated byreference, describes an improved scanning field emission microscopeinstrument. A field emission tip 11, which is a suitably shaped cathodeof appropriate metal, and described in the aforementioned article,produces the beam of charged particles when placed in a sufficientlyhigh electric field. In the scanning electron microscopy systems, thecharged particles are of course electrons. However, in a field ion typedevice, the tip acts as a source of ions issuing from a virtual sourcewithin the structure of the field emission tip 11. Tip voltage unit 17interconnected between field emission tip 11 and a first anode structure12 supplies the necessary field gradient for production of the chargedparticles. The acceleration voltage unit 16 interconnected between fieldemission tip 11 and a second anode 13 further downstream provides thenecessary acceleration forces and in conjunction with anode l2 focusesthe particles to a desired beam size. This type of gun structuredescribed essentially constitutes a self-focusing unit without need forfurther lensing systems unless peculiar resolution requirements areplaced upon it. A beam aperture plate 14 shown distally located of thesecond anode 13 may be appropriately located in any number of positionsin the gun and essentially determines the shape and size of the chargedparticle beam.

Since it is a scanning type device to which our attention is drawn, itis necessary that the charged particle beam be deflected in apredetermined pattern in order to appropriately scan the specimenundergoing ingestigation. Deflection coils 15 are driven by the scan andSync control unit 32 to deflect the beam in the appropriate manner. Inthis system, the beam is scanned horizontally across the specimen, thenrapidly returned to a starting point vertically displaced from theinitiaLand subsequent lines are then scanned in a similar manner. As thehorizontal beam is swept across the specimen it is continuouslyvertically displaced, thusly eventually covering the entire specimensurface. This type of scanning pattern is well known to those in themicroscopy art and is entirely similar to that normally used in thetransmission of television pictures.

The beam after deflection is passed through an interconnecting orifice18 between gun It) and specimen chamber 20. The specimen 21 is locatedin the physical path of the electron beam and is scanned in the mannerheretofore described. Impingement of the beam upon the specimen 2]produces the emission of secondary particles,in the specific instance ofthis embodiment secondary electrons as well as reflected electrons whichmay be detected by unit 22. The sensing device 22 is of thescintillation type well known to those ordinarily skilled in the art andconverts the signals derived from the specimen 21 to an amplifiedelectrical signal upon which detector unit 31 may further operate. Thesensing unit 22 in addition to the scintillator detector normallyincludes a photomultiplier or other similar e1- ement. The detector 31further amplifies the signal and appropriately modifies it for operationwith a viewing tube of the normal TV monitoring type. This detectormeans of the display system may then be thought of as incorporatingsensing unit 22 as well as detector 31.

In the usual viewing tube 30, a viewing face 35 is provided having asensitized surface which emits light upon the impingement of an electronbeam. Normally, the sensitized surface is coated with a phosphor orother suitable material. The beam of the viewing tube is generated froma thermionic type cathode and deflected by electrostatic platestypically shown as 33 of viewing tube 30. If the image formed on theface of viewing tube 30 is to have coherency with the signals derivedfrom the specimen 21, it is essential that a fixed and knownrelationship in both time and phase be maintained between the fieldemission gun 10 electron beam and the electron beam of viewing tube 30.Normally, it is found suitable to have both beams in exact synchromismwith the signal produced by detector 31, thus producing a real timeimage of the specimen being investigated. As previously alluded to,advantage may be taken of viewing tube 30 face persistency as well asthe persistency of the eye in limiting the bandwidth requirements of aviewing system. A complete vertical and horizontal sweep of the surfaceof a specimen is known as a field. If the field contains all of thehorizontal scanning lines necessary to achieve system resolution, thenthe term field and frame are equivalent within a given system. However,in those instances where interlacing is used, i.e., each field containsonly a portion of the total number of lines in the pattern, then anumber of fields are required to make up or produce a complete frame. Inthe instance of a two-to-one interlace ratio, there are two fields perframe while in the case of a four-to-one ratio, there are four fieldsper frame. In this embodiment, a total of 1,155 scanning lines has beenfound adequate to achieve a 250 Angstrom resolution system. This meansthere are approximately 281 lines per field. The first field is paintedby the tube beam on its sensitized face leaving a space equivalent tothree lines between each succeeding line of the field. The nextsubsequent field paints a second line while the third and fourth fieldsfill in the remaining two lines of the pattern, thus after completion offour fields a complete picture having maximum resolution is obtained onthe face of viewing tube 30. The Sync signals necessary to keep thescanning lines, both vertical and horizontal of viewing tube 30 anddeflection coils 15, in exact phase relationship are incorporated intothe output signals of the Scan and Sync control unit 32. The outputsignal of detector 31 applied to a signal grid 34 of viewing tube 30modulates the electron beam of the tube and thereby affects thebrightness and contrast of the image to produce a recognizable pictureof specimen 21.

With reference to FIG. 3, there is shown a portion of a normal rasterhaving a four-to-one interlace ratio. The numbers shown in FIG. 3pertain to time sequencing or order of the fields making up a completeframe. In anormal display system, the second field lines are laid downin the next physical location on the face of tube 30. Each succeedingfield is again laid down in the next available allocated line space andthusboth the time sequencing and position order of the field lines arein phase. As previously indicated, it is just this order which producesin the eye of the observer a cascading or waterfall effect in the image.This effect is unduly wearisome to the observer and results in loss ofeffective resolution to the eye. The applicants have discovered that aphysical interruption of the sequencing pattern of the scanning fieldseliminates this cascade effect and produces a substantially stationaryimage to the eye of the observer. FIG. 2 demonstrates a sequencingpattern found to produce the desired results. The first field of theframeis laid down in anormal manner. The second field, however, is laiddown in the position where the normal third field would be found, thethird field is placed where the fourth field would normally be locatedand the fourth field is placed in the remaining slot or position of theraster. Stating this somewhat more succinctly, a normal field orderwould correspond to l, 2, 3, 4, 1, et seq, while the raster of theapplicants display system would follow, for example, a pattern 1, 4, 2,3, 1, et seq., thereby preventing any more than two succeeding fieldlines from appearing in the next physical order. Another pattern foundappropriate to the applicants purpose is 1, 3, 2, 4, and 1, et seq. Theachieving of such horizontal scanning patterns is a rather complexelectronic problem involving, however, only ordinary skill in the art.Teachings on this subject adequate to accomplishment of the purpose maybe found in many texts, for example, Pulse And Digital Circuits" byJacob Millman and Herbert Taub, McGraw-Hill Electrical and ElectronicEngineering Series, 1956, Chapter 17, entitled PULSE AND DIGI- TALSYSTEMS. Of course, as the interlacing ratio is increased, the selectionof scanning signals and their sequence is further complicated but againwell within the ordinary skill of the art.

The applicants have thus provided a reliable low-cost field emissionscanning microscopy display system having minimum bandwidth requirementsand alffording a substantially stationary image to the eye of theobserver. It is intended that the embodiments described herein beillustrative of the invention and that those modifications apparent toone skilled in the art be included within the scopg ofthe invention.

We claim:

I. In a scanning electron microscope comprising a charged particle gunfor generating a beam of charged particles directed to impinge upon aspecimen, scanning means to cause said beam to scan a surface of saidspecimen, detection means responsive to and providing an outputproportional to charged particles leaving the surface of said specimen,display means responsive to said detection means for displaying an imageof the charged particles detected by said detection means wherein theimprovement comprises:

control means electrically connected to said scanning means adapted tocause said scanning means to scan said specimen surface in a field linepattern having at least a three-to-one interlacing;

interlacing control means in said control means to cause the interlacingorder of said field lines to be other than sequential order;

display means including a cathode ray tube, the cathode ray of which isadapted to be modulated by said output of said detection means anddeflected synchronously with said beam of charged particles whereby saidbeam of charged particles and said cathode ray are synchronized so as toscan said specimen and tube in lines which are uniformlynon-sequentially interlaced from field-to-field which therefore,produces a high resolution picture substantially free of the phenomenonof cascade effect to the eye. 2. The improvement according to claim 1wherein said charged particle gun includes a field emission tip, a firstfield anode and a second anode and said field and second anode ,formaccelerating and focusing anodes for the electron'beam generated by saidfield emission tip.

3. The improvement according to claim 2 including control means adaptedto cause said scanning means to scan said specimen surface in aninterlacing ratio of four-to-one and said interlacing control meansinhibits the occurrence of more than two succeeding field lines in nextsucceeding adjacent locations in said scan pattern.

4. The improvement according to claim 3 wherein said interlacing controlmeans is adapted for a sequence of l, 4, 2, 3, and 1 et seq.

5. The improvement according to claim 3 wherein said interlacing controlmeans is adapted for the sequence of l, 3, 2, 4, and 1 et seq.

6. The improvement according to claim 1 wherein said detection meansincludes a scintillation type detector and photosensitive means forconverting light produced by said scintillation detector to anelectrical signal proportional thereto.

7. The improvement according to claim 1 wherein said charged particlegun is adapted to be an ion source including a field emission tip, afirst field anode, a second anode, an ionizable gas disposed about saidtip, and said field and second anode form accelerating and focusinganodes for the ions generated at said emission tip.

1. In a scanning microscope comprising a charged particle gun forgenerating a beam of charged particles directed to impinge upon aspecimen, scanning means to cause said beam to scan a surface of saidspecimen, detection means responsive to and providing an outputproportional to charged particles leaving the surface of said specimen,display means responsive to said detection means for displaying an imageof the charged particles detected by said detection means wherein theimprovement comprises: control means electrically connected to saidscanning means adapted to cause said scanning means to scan saidspecimen surface in a field line pattern having at least a three-to-oneinterlacing; interlacing control means in said control means to causethe interlacing order of said field lines to be other than sequentialorder; display means including a cathode ray tube, the cathode ray ofwhich is adapted to be modulated by said output of said detection meansand deflected synchronously with said beam of charged particles wherebysaid beam of charged particles and said cathode ray are synchronized soas to scan said specimen and tube in lines which are uniformlynon-sequentially interlaced from field-tofield, producing a highresolution picture substantially free of the phenomenon of cascadeeffect to the eye.
 2. The improvement according to claim 1 wherein saidcharged particle gun includes a field emission tip, a first field anodeand a second anode and said field and second anode form accelerating andfocusing anodes for the electron beam generated by said field emissiontip.
 3. The improvement according to claim 2 including control meansadapted to cause said scanning means to scan said specimen surface in aninterlacing ratio of four-to-one and said interlacing control meansinhibits the occurrence of more than two succeeding field lines in nextsucceeding adjacent locations in said scan pattern.
 4. The improvementaccording to claim 3 wherein said interlacing control means is adaptedfor a sequence of 1, 4, 2, 3, and 1 et seq.
 5. The improvement accordingto claim 3 wherein saiD interlacing control means is adapted for thesequence of 1, 3, 2, 4, and 1 et seq.
 6. The improvement according toclaim 1 wherein said detection means includes a scintillation typedetector and photosensitive means for converting light produced by saidscintillation detector to an electrical signal proportional thereto. 7.The improvement according to claim 1 wherein said charged particle gunis adapted to be an ion source including a field emission tip, a firstfield anode, a second anode, an ionizable gas disposed about said tip,and said field and second anode form accelerating and focusing anodesfor the ions generated at said emission tip.